Electronic messages have become an indispensable part of modern communication. Electronic messages such as email or instant messages are popular because they are fast, easy, and have essentially no incremental cost. Unfortunately, these advantages of electronic messages are also exploited by marketers who regularly send out unsolicited junk messages (also referred to as “spam”). Spam messages are a nuisance for users. They clog people's email box, waste system resources, often promote distasteful subjects, and sometimes sponsor outright scams.
To reach a broad audience, the senders of spam (also referred to as “spammers”) usually generate a huge amount of messages using automated computer programs. There are many existing techniques for filtering spam messages, some of which attempt to identify machine generated messages and block them. Upon receiving a message, the recipient may issue a challenge that is difficult for a computer to answer but easy for a person. For example, the challenge may include a picture of three objects, with the question “how many objects are in the picture?” The sender needs to choose three from a number of available answers and send the answer to the recipient. This type of question, referred to as a human answerable question, is difficult for machines to answer correctly but easy for human beings. The reply is then examined by the recipient's computer. The message is delivered to the recipient if the answer is correct, and processed as spam otherwise. This type of challenge/response scheme incurs significant overhead in for the sender of the email, making email no longer “free” and therefore more difficult for the spammers.
The challenge/response scheme described above is effective for blocking spam messages, but has many drawbacks. For example, since the message is not delivered until the challenge is answered, long delays may result if the sender is not available to answer the challenge right away. Also, the scheme is inconvenient for legitimate users that need to send out messages to new addresses often, for example a business that has many new clients. It would be desirable to have the benefit of the challenge/response scheme for spam blocking purposes, but does not result in long delays in message delivery or excessive burden for the sender.